1. Field of the Invention
This invention relates generally to packaging arrangement for a sensor. In particular the invention relates to a packaging arrangement including a case for a micro-machined-semi-conductor sensor die which establishes electrical contact between electrical leads of the sensor die and external contacts of the case, and for immobilizing the die within the case. Still more particularly the invention relates to a packaging arrangement in which a sensor die is electrically connected to outside contacts and secured within an evacuated cavity of a case without organic materials of any kind inside the cavity.
2. Description of the Prior Art
The sensor die to be packaged is a micro-machined device manufactured from silicon and arranged and designed to measure acceleration along a principal axis. The sensor includes a moving proof silicon mass supported from a surrounding frame of silicon by eight supporting springs on top and bottom sides of the proof mass. The proof mass moves between top and bottom caps which face top and bottom surfaces of the proof mass, but are spaced therefrom. The top and bottom caps are secured to the surrounding support frame from which the proof mass is supported. The sensor die is described in U.S. Pat. No. 5,652,384 and in pending U.S. application Ser. No. 08/516,501, both of which are incorporated by reference.
Acceleration of the support frame causes the proof mass to move vertically with respect to the top and bottom caps and the support frame. The variation in distance between the proof mass and the top and bottom caps changes the capacitance between the proof mass and the caps, and an electrical signal corresponding to such capacitance is measured by the sensor. In a "nutshell", the distance between the top and bottom caps, and the surrounding silicon support frame must be precisely controlled and remain stable in order to obtain the extremely high sensitivity requirements of the sensor.
Thus, for extreme sensitivity requirements, the silicon frame and the silicon top and bottom caps to which electrical conductivity paths must run to outside contacts of the package, must remain dimensionally stable, so as not to induce error signals due to extraneous changes in the dimension of the frame and the top and bottom caps with respect to the proof mass.
The sensor die to be packaged is designed to be sensitive to extremely small forces, e.g., one millionth (10.sup.-6) of a gravitational unit "g". As a reference comparison, a typical automobile airbag micro-machined accelerometer requires only one tenth (10.sup.-1) of a gravitational unit "g" sensitivity. While the sensor must be capable to measure extremely small accelerations, it also must be able to withstand extremely high shocks in excess of 5000 g.
The sensor to be packaged is to be used for seismic measurements in the exploration for hydrocarbon deposits, and for downhole positioning devices for oil and gas well drilling and for inertial guidance systems. Each of such applications calls for extreme sensitivity in order to expand the boundaries of the measurements of those specialities. Furthermore, the physical environment in which the sensor must work is extreme. The sensor must survive high shocks and wide temperature ranges of from -40.degree. C. to +85.degree. C.
Because of the high sensitivity requirement, the sensor must be packaged in a high vacuum (e.g., 10.sup.-3 mm of Hg for 10 years) in order to reduce an effect called Squeeze Film Damping and in order to increase the quality of the signal. Squeeze Film Damping is a damping phenomenon caused if air is trapped between two closely separated surfaces. The distance between top or bottom surfaces of the proof mass and the top and bottom caps of the die to be packaged for capacitive measurements is about one to two micrometers (10.sup.-6 meter) in distance.
Prior packaging arrangements have required some sort of die attach material to affix the sensor die to a case or enclosure. For example, solder has been used in prior art packaging methods to secure the die to a surrounding enclosure. However, the difference in the thermal expansion of different materials (silicon support frame, solder attach material and ceramic packages) can induce strain to the silicon electrode surface thereby distorting the accuracy of the acceleration output signal especially where specifications of -40.degree. C. to +85.degree. C. operating temperatures are required.
Vacuum packaging requirements also creates problems not solved by the prior art. Prior art packaging methods typically use epoxy, RTV or organic adhesives to fix a sensor die to a case. But the use of such adhesive materials, where a high vacuum must exist for the die inside the enclosure, causes outgassing of such material inside the evacuated enclosure. Outgassing causes molecules to be added to the space inside the enclosure thereby decreasing the vacuum level inside the package.
Prior packaging arrangements have also typically required bonding materials for electrical connections or bond wires between contacts of the sensor die and external contacts of the package. However, in order for bond wire to install and function properly, the sensor die has to be affixed to the package with solder or other adhesive material that can either cause outgassing or induce stress to the die.
3. Identification of Objects of the Invention
A primary object of the invention is to provide a packaging arrangement which secures a micro-machined silicon sensor die to an enclosure without the use of attach materials between the die and the enclosure.
Another object of the invention is to provide a securing arrangement for a sensor die within an enclosure while providing high vacuum of the die inside the enclosure and minimizing the chance for outgassing of materials inside the enclosure.
Another object of the invention is to secure a sensor die within a ceramic package enclosure while providing connections to the sensor from electrical contacts on the exterior of the ceramic package without the use of organic materials.
Another object of the invention is to provide a secure coupling between a sensor die and an enclosure which prevents relative movement when the enclosure is subjected to high shock loads.
Another object of the invention is to provide a high vacuum level of the die while enclosed in its case and to maintain such vacuum level throughout a long operating life of the sensor.
Another object of the invention is to provide a packaging arrangement for an extremely sensitive acceleration sensor which is isolated from strain due to differences in thermal expansion between the silicon of the sensor and all the materials which connect the sensor to external electrical contacts of the package.
Another object of the invention is to provide an accelerometer/enclosure arrangement which is rugged, simple to install, and reliable under conditions of wide variations of operating temperatures, high vacuum inside the enclosure and high external shock.